Immunoglobulins did not arise in evolution to fight infection. Contemporary immunology focusses primarily on the complex interactions between B cells and T cells in response to external antigens, which lead to the secretion of immunoglobulins as a defence reaction to infection by bacteria and viruses. This book argues that these phenomena may nevertheless be relatively late evolutionary developments, due to the redeployment of a system invented for other reasons. Variable-region molecules (VRM), including circulating immunoglobulins, exist in all vertebrates from sharks to humans; but they are completely absent from all invertebrates. This presents an evolutionary enigma: it is almost as though the VRM system never existed in a primitive form, but arose fully-fledged at the time of the first vertebrates. This book is a speculative attempt to resolve this enigma by reconstructing the missing links which led to the appearance of a primordial VRM system, through an endogenous, self-organizing process. It is suggested that this primordial VRM system instituted a molecular ecology, a function so important that from then on no vertebrate has been able to do without it.